1. Field of the Invention
The present invention relates to an object detection sensor suitably used as, for example, a switch for opening and closing operations of a trunk of a vehicle.
2. Description of the Related Art
An object detection sensor for detecting a contact thereof with an object such as a human body (including an approach to the object in a very narrow space) is widely used as an operation switch of various apparatuses.
A common touch sensor such as the one shown in FIG. 8A has heretofore been known as the object detection sensor of the above type. In the touch sensor, a touch electrode 2 is disposed on a surface of a front face portion (a portion for detecting the contact) of a sensor case 1 and connected to a detection circuit which is formed on a circuit substrate 3 provided in the sensor case 1 via a wiring 4 penetrating the front face portion of the sensor case 1. The reference numeral 5 denotes a housing for an apparatus to which the sensor is mounted, and the housing usually has a ground potential. The detection circuit detects the contact based on a change in potential or magnetic field which is caused by the contact of the touch electrode 2 with the human body or the like.
Also, as a sensor wherein a detection electrode is not exposed, a sensor of electrostatic capacitance type disclosed in JP-A-6-162889 is known. As shown in FIG. 8B, in the sensor of this type, a detection electrode 7 (detection element) is disposed on an inner surface of a front face portion of a sensor case 6 (surface material) and connected to a detection circuit which is on a circuit substrate 8 provided in the sensor case 6 via a wiring 9 in the sensor 6. In addition, the detection circuit detects the human body or the like based on a change in floating capacitance which is formed by the detection electrode 7 (electrostatic capacitance between the detection electrode 7 and the ground) and caused by an approach of the human body (an approach to the extent that the human body contacts with the front projection portion of the sensor case 6). Further, the sensor case 6 is projected frontward (upward in FIG. 8B) in order to prevent the front face side of the detection electrode 7 from being wet with water.
The conventional object detection sensor described above has the following problems.
Since it is necessary to form the wiring 4 in such a fashion as to penetrate the front face portion of the sensor case 1, the object detection sensor shown in FIG. 8A has a drawback of bad assembly efficiency. Also, since the touch electrode 2 serving as the detection electrode is exposed, it is highly likely that malfunction is caused due to water wetting (for example, water wetting which provides conduction between the housing 5, that is, the ground, and the touch electrode 2), thereby making it ultimately difficult to use the object detection sensor in an environment where water drops are scattered, such as in the rain.
In turn, since the detection electrode of the electrostatic capacitance type sensor such as the one disclosed in JP-A-6-162889 is not exposed, the electrostatic capacitance type sensor is free from the above-described drawback. However, the floating capacitance which is the input signal to the detection circuit changes by a large scale depending on a contact area (area of the portion opposed to the detection electrode 7) of the human body (usually the fingertip) contacting with the detection electrode 7 and the change in floating capacitance is remarkably reduced when the contact area is small (for example, in the case where the fingertip slightly touches the detection electrode 7 as indicated by the dotted line in FIG. 8B). Therefore, the electrostatic capacitance type sensor has a drawback of failing to detect the human body or the like without a considerable degree of increase in sensitivity of the detection circuit. Further, in the case where the sensitivity of the detection circuit is increased so as to reliably detect the human body or the like even when the contact area is small, it is highly likely that the malfunction is caused by water drops and the like. Since it is impossible to increase the sensitivity of the detection circuit in the environment where water drops are scattered or in the environment where there is too much noise due to other dielectric contaminants, the electrostatic capacitance type sensor fails to detect the human body or the like without a sufficient contact area in the end. Thus, the electrostatic capacitance type sensor is insufficient in terms of usability and responsiveness. As a method of increasing the sensitivity of the detection circuit, it is known to set a small difference between a threshold value of the detection and an input signal (a margin of a threshold value) during the non-contact period in the detection circuit or to increase an amplifying ratio of the input signal in the detection circuit. However, with both of the method, the malfunction tends to occur due to the water drops and the noise of other dielectric contaminants or the temperature drift. In the case of increasing the amplifying ratio, the power consumption of the detection circuit is disadvantageously increased.